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Limited information is available on frequency and seasonal timing of irrigation in perennial ryegrass seed crops. Two field experiments were conducted in Oregon to investigate irrigation effects on seed production in seven cultivars of perennial ryegrass. Three spring irrigation treatments were tested in Experiment 1 over a two-year period: none (control), single irrigation to fill the soil profile to field capacity (FC) at anthesis, and multiple irrigations to maintain soil water deficit ≤50 mm below FC until the beginning of seed fill; and single fall and spring irrigation timings were tested in Experiment 2 over a three-year period. Spring irrigation increased seed yield of perennial ryegrass in both experiments while fall irrigation had no effect in the second experiment. A single spring irrigation increased seed yield by an average 16.1% whereas multiple irrigations resulted in an average increased yield of 24.6% in Experiment 1. Single spring irrigation in Experiment 2 increased seed yield by an average 6.5%. Seed yield increases attributable to spring irrigation were the result of a combination of increased seed number and seed weight. Spring irrigation increased harvest index and seed set. Water use efficiency was reduced by spring and fall irrigation. Perennial ryegrass seed crops can benefit from spring irrigation but the frequency of irrigation is best determined by the quantity of water available and economic considerations. [ABSTRACT FROM AUTHOR]